The invention relates to a system for measuring loading, stresses and/or material fatigues occurring in a structure. The invention also relates to a measuring unit and a measuring sensor suitable for use in connection with said system. The invention is applicable especially to the measurement of stresses and loading in a ship hull.
Load exerted on a ship hull must be continuously measured for any changes taking place in the structure, such as material fatigue, to be detected before the structures break. Similar measurements are performed regarding other structures, such as other vehicles, bridges and buildings.
There are conventional methods comprising the fastening of measuring sensors to a ship hull to measure local deformations of the structure. FIG. 1 shows a top view of the main deck 10 of a ship hull and the typical positioning of measuring sensors 11, 12, 13 and 14 on the main deck of the ship hull. Typically used measuring sensors are steel bars with an approximate length of 1-2 meters, which are solidly attached e.g. by welding to the main deck of the ship hull, with one end of the bar stationary and the other end axially movable. At the junction, e.g,. a movement sensor is disposed to measure the mutual movement. The bar of the measuring sensors shall have a length at least equal to the length mentioned above for the relative movements of the bars to be sufficiently large (typically a maximum of +/xe2x88x925 mm) and the deformation measurement to be sufficiently accurate.
In conventional methods, the weak signal obtained from the measuring sensors is conducted to the central unit of the measuring system, where the signals received from the sensors are amplified and converted into digital form, and mathematical signal processing is performed. There are different standard signal processing models to allow optimally reliable detection of critical deformations by means of measurement results. Such models are i.a. the frequency range analysis and the so-called Rainflow classification. The Rainflow classification is defined i.a. in ASTM Standard E1049-85 (Reapproved 1990).
The prior art systems mentioned above involve some drawbacks. Firstly, the systems have been devised for a given number of standard measuring sensors, so that additional measuring sensors cannot be provided if necessary. This is due to the fact that the computing capacity of the central processing unit of the system and the interfaces have been dimensioned for a predetermined number of measuring sensors. There are situations where the usual number of measuring sensors will not be sufficient, such as for instance in a catamaran ship, where a considerable greater number of measuring sensors, e.g. 60 units, might be necessary, compared with a xe2x80x9cconventionalxe2x80x9d ship hull, where there are usually e.g. 4 measuring sensors.
Secondly, prior art systems involve the problem of interference connected to the long measuring lines between the measuring sensors and the central processing unit. This is difficult to avoid by protecting the measuring cables, because the signals to be transmitted from the measuring sensors are low-frequency analogue signals.
In addition, prior art measuring sensors also involve the problem of onerous calibration. On reason for this is the cumbersome detachment of large measuring sensors from the ship hull, and another reason is that a large-sized measuring sensor may behave differently when attached to a ship hull than under calibration conditions. Consequently, accurate and reliable calibration will be difficult to achieve. The replacement of a defective measuring sensor is also awkward.
The object of the present invention is to avoid the prior art problems mentioned above with the aid of a new measuring solution.
One inventive idea is that the measuring sensor unit, i.e. measuring unit of a measuring system, comprises means for processing a signal obtained from the sensor so that complete mathematical measurement results can be transmitted from the measuring unit to the central processing unit. In this manner, no large computing power will be required in the central processing unit, nor does this capacity requirement depend substantially on the number of measuring sensors used. Thus the number of measuring sensors can be readily increased. In addition, the negative impact of interference connected to the cable between the measuring unit and the central processing unit will be small, because the signals to be transmitted are digital and the transmission can be repeated if necessary.
A second inventive idea is that the measuring sensor has mechanical features allowing it to be readily attached to and detached from the structure to be measured. In this case, the structure to be measured preferably includes a base for attachment of the measuring sensor. The measuring sensor can then be calibrated apart from the structure to be measured and the behaviour of the measuring sensor will be exactly the same in the structure to be measured as under calibrating conditions.
A further inventive idea is to form a measuring sensor from a sensor assembly and a strain gauge so that deformations are transmitted to the strain gauge, which is attached to an elastic area in the sensor assembly. The elastic area is preferably formed with a double H -opening in the sensor assembly. In addition, the sensor preferably comprises a second strain gauge for temperature calibration, which is attached to a part of the sensor assembly where no deformation occurs. In this manner, a small-sized but high-precision measuring sensor will be provided, which is easy to handle e.g. during calibration.
The system of the invention for measuring loading on a structure, which comprises a central processing unit and at least one measuring unit, is characterised by the measuring unit comprising
a measuring sensor for converting structure deformations into an electric signal,
means for converting said signal into a digital signal,
means for mathematical processing of the digital signal, and
means for transmitting the processing results to the central processing unit, the central processing unit comprising means for receiving and collecting processing results transmitted from at least one measuring unit.
The measuring sensor of the invention for measuring loading on a structure is characterised by the fact that the measuring sensor comprises a sensor assembly, which is attached to the structure to be measured and includes two rigid members for attachment to the structure to be measured and an elastic member between these, a first strain gauge having been fastened to said elastic member for transmitting deformations of the structure and the sensor assembly to the strain gauge with a view to generate a signal proportional to the deformations of the strain gauge.
The measuring unit of the invention for measuring loading on a structure is characterised by the fact that the measuring unit comprises
a measuring sensor for converting deformations of a structure into an electric signal,
means for converting said signal into a digital signal,
means for mathematical processing of the digital signal, and
means for transmitting the processing results to the central processing unit. Preferred embodiments of the invention are described in the dependent claims.
The invention is explained in greater detail below with reference to the accompanying drawings, in which
FIG. 1 shows conventional positioning of measuring sensors on the deck of a ship hull,
FIG. 2 is a block diagram of a measuring system of the invention,
FIG. 3 is a block diagram of a measuring unit of the invention,
FIG. 4 is a flow diagram of a procedure of the invention for transmitting measuring data,
FIG. 5a is a top view of a measuring sensor of the invention and
FIG. 5b is a lateral view of a measuring sensor of the invention.